Interpretive Summary: Avocado is a commercially valuable fruit crop cultivated in tropical and subtropical climates throughout the world. Scientists currently recognize three horticultural races of avocado: Mexican, Guatemalan, and West Indian. Previous research found that chemical differences among avocado leaves were not reliable for distinguishing the three genetic races. Scientists at the USDA-ARS Subtropical Horticulture Research Station (Miami, FL) developed new methods for sampling and analyzing a particular class of chemicals (sesquiterpenes or C15 hydrocarbons) from avocado trees. They used those methods to (1) document how sesquiterpenes are distributed throughout the tree from leaf to trunk, and (2) show that there are chemical differences that can separate avocado races when samples are collected from branches at least 2.5 cm (1 inch) in diameter. This information will benefit plant taxonomists and geneticists. In addition, information on sesquiterpene content will help scientists identify the host-based attractants used by a new avocado pest, the redbay ambrosia beetle (RAB). This wood-boring insect spreads laurel wilt, a deadly fungal disease that was found in Florida avocado groves in 2012. Identification of attractants will facilitate development of improved lures for RAB. This will allow action agencies to detect RAB earlier, slow the spread of laurel wilt disease, and potentially provide the basis for pest control strategies.

Technical Abstract:
Avocado is a commercially valuable fruit crop cultivated in tropical and subtropical climates throughout the world. Taxonomists recognize three horticultural races of avocado, consisting of Mexican (Persea americana var. drymifolia), Guatemalan (P. americana var. guatemalensis), and West Indian (P. americana var. americana) varieties. Published research that attempted to differentiate among the horticultural races by using leaf chemistry found that sesquiterpene content was only useful for discrimination of pure Mexican from Guatemalan and West Indian, but not to distinguish between Guatemalan and West Indian races. This study presents a sampling method for analysis of sesquiterpenes from avocado leaf, branch and trunk samples. Our results indicate that sesquiterpene content from leaves and small diameter branches (< 2.5 cm) was highly variable; however, sesquiterpenes were much less variable within wood from larger diameter branches and trunk samples, providing information representative of avocado varietal differences. In addition to chemotaxonomic applications, information on sesquiterpene content of avocado wood is needed for identification of host-based attractants for a new avocado pest, the redbay ambrosia beetle (Xyleborus glabratus). This insect vectors a fungal pathogen that causes laurel wilt, a lethal vascular disease that currently threatens avocado production in south Florida. Females of X. glabratus identify appropriate host trees based upon emissions of terpenoids, particularly a-copaene. Our results are discussed in terms of how proximo-distal distributions of sesquiterpenes may function as host-location cues by this wood-boring pest.